In recent years, utilization of biomass as an alternative material to petroleum has been drawing attention due to concerns about depletion of petroleum resources and global warming. In particular, methods of obtaining sugars by hydrolysis of biomass containing polysaccharides such as starch and cellulose have been actively studied. This is because various chemical products can be produced by microbial fermentation using sugars as feedstocks.
A cellulose-derived sugar liquid contains fine particles such as lignin, tannin, silica, calcium and undegraded cellulose; water-soluble macromolecules such as oligosaccharides, polysaccharides, tannin and enzymes; and low-molecular weight fermentation inhibitors; as impurities. In a known method of removing these impurities, fine particles are separated into the feed side by a microfiltration membrane, water-soluble macromolecules are separated into the feed side by an ultrafiltration membrane, and fermentation inhibitors are removed from the feed side by a nanofiltration membrane or reverse osmosis membrane (WO 2010/067785).
It could therefore be helpful to provide a method of producing a cellulose-derived sugar liquid using a separation membrane, wherein the separation membrane clogged with impurities in the cellulose-derived sugar liquid, especially with water-soluble macromolecules, is washed, thereby providing a method for effectively removing impurities from a cellulose-derived sugar liquid.